Opening speed, or delay, defines the relationship between the percentage by which the fuse or circuit breaker is operating over its Amperage Rating and the length of time that will be required for it to open. The opening of a fuse or circuit breaker is determined not just by the amperage rating, but by the amount of time and the percentage over its amperage rating at which it is being operated. The higher the % of current flow to amperage rating, the faster the circuit protector will open.

A chart such as the one shown here commonly represents this relationship.